This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 99-61330 filed in Korea on Dec. 23, 1999; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a phosphor with superior emission efficiency at low voltages, and more particularly, to a yttrium silicate based phosphor having effective emission at low voltages, which is for utility in field emission devices (FEDs), and a method for synthesizing the same.
2. Description of the Related Art
During application of a low voltage to a device, due to the short infiltration length of electrons into a phosphor screen and rapid non-luminescent recombination speed at the surface of the phosphor screen, the luminescent efficiency of phosphor is considerably lowered. To operate a FED with a driving voltage as low as 1 kV or less, there is a need for a phosphor that exhibits a high luminescent efficiency at a low voltage.
A sulfide-based phosphor, which was used for cathode ray tubes (CRTs), has been modified for use in FEDs. Such sulfide phosphors show effective emission at a high voltage of 5 to 10 kV. However, in application to FEDs operated with a driving voltage, there occur the problems of reduced chemical stability and charge build-up. To improve these problems, extensive research has been carried out on oxide based phosphors, particularly on yttrium silicate based phosphors, which are considered to be an excellent phosphor.
Although yttrium silicate based phosphors have better chemical and thermal stabilities than sulfide based phosphors, chromaticity and luminance are very low. Reportedly, a sulfide based green phosphor (ZnS:Cu,AI) has chromaticity (x=0.333, y=0.614), an oxide based phosphor (Y2SiO5:Tb) has chromaticity (x=0.333, y=0.582). Also, the luminance of the oxide based phosphor is about 30-40% of the luminance of the sulfide based phosphor.
A major challenge in applying oxide based phosphors to FEDs operating with a low voltage is the modification of Y2SiO5:Tb phosphor for production of superior phosphors with improved luminance and chromaticity. As a result of the efforts, some patents related to the fabrication of such phosphors have been issued. For example, EP0060647 (Sep. 22, 1982) disclosed addition of zinc (Zn) in the amount of 0.46 to 1 moles to a yttrium silicate phosphor. GB2177714 (Jan. 28, 1987) introduced variation of flux. GB1452180 (Oct. 13, 1976) disclosed addition of fluoride (NH4F, YF3 and BaF2) based flux.
To solve the above problems, it is an objective of the present invention to provide a yttrium silicate based phosphor with superior luminance and chromaticity at low voltages, which is synthesized by modifying Y2SiO5:Tb extensively being used as a phosphor for field emission displays (FEDs).
It is another objective of the present invention to provide a method for synthesizing a yttrium silicate based phosphor with superior luminance and chromaticity at low voltages.
In one aspect, the present invention provides a yttrium silicate based phosphor having effective emission at low voltages, having the general formula
(Y1xe2x88x92kLnk)2(Si1xe2x88x92yPy)O5xc2x7xZnO
where Ln is a lanthanide element selected from gadolinium (Gd), Tb, Ce and Eu, and k, y and x are rational numbers in the range of 0xe2x89xa6kxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and 0xe2x89xa6xxe2x89xa61.
Preferably, in the above formula, k is in the range of 0.0005 to 0.2, x is in the range of 0 to 0.1, and y is in the range of 0.0001 to 0.01.
In another aspect, the present invention provides a method for synthesizing a yttrium silicate based phosphor having effective emission at low voltages by a coprecipitation method, comprising the steps of: (a) dissolving Y2O3, and Eu, Tb or Ce as an activator, and ZnO as a sensitizer in a nitric acid solution, adding SiO2 powder to the solution, stirring the mixture, and adding the mixture to an oxalic acid aqueous solution to precipitate; (b) filtering the precipitate from step (a) and drying the filtered precipitate; (c) adding a flux to the dried precipitate, firing the mixture at a temperature of 900 to 1000xc2x0 C. to produce a phosphor; (d) firing the phosphor from step (c) at 1400xc2x0 C. in an oxidation atmosphere; (e) firing the phosphor from step (d) at a temperature of 1200 to 1300xc2x0 C. in a reduction atmosphere.
Preferably, the SiO2 powder added in step (a) has a particle size of 0.007 to 0.07 xcexcm.
In another embodiment, a method for synthesizing a yttrium silicate based phosphor having effective voltages at low voltages by a solid state reaction method, comprises the steps of: (a) mixing Y2O3 and SiO2 powder as host substances, eurobium (Eu), terbium (Tb) and cerium (Ce) powder as an activator, and ZnO powder as a sensitizer; (b) adding a flux to the mixture from step (a) and firing the mixture at a temperature of 900 to 1000xc2x0 C. to produce a phosphor; (c) firing the phosphor from step (b) at 1400xc2x0 C. in an oxidation atmosphere; and (d) firing the phosphor from step (c) at a temperature of 1200 to 1 300xc2x0 C. in a reduction atmosphere.
Preferably, the SiO2 powder added in step (a) has a particle size of 0.007 to 0.07 xcexcm.